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flows have covered the region. Later erosion has partly uncovered the
older sediments and intrusives.
The ores are principally magnetite and hematite with a small amount
of limonite, and occur (a) as fissure veins in the andesite, (6) as fissure
veins and replacement deposits along the contact of the andesite and lime-
stone, and (c) as a cement in a Cretaceous quartzite-breccia.
J. C. J.
Geology of the Rangeley Oil District, Colorado. By Hoyt S. Gale.
Bulletin 350, U. S. Geological Survey.
A small field at the western border of Colorado is described where con-
siderable prospecting for oil has been going on with some success. The
rocks are principally Cretaceous and Tertiary. The base of the Wasatch
formation (Tertiary) rests with apparent conformity upon the top of the
Mesaverde formation (Cretaceous), but the absence of formations found
between them elsewhere in Colorado indicates a non-conformity here.
The structure is a quaquaversal fold with little evidence of faulting. The
oil occurs presumably in lenses in the Mancos (Cretaceous) shale.
J. C. J.